Seasonal orographic effect of North American Mountain Range at different levels and its remote control on tropical climate

Orography significantly influences global climate patterns. Previous studies show the North American Mountain Range (NAMR) impacts regional climates seasonally but have not thoroughly illustrated the seasonally different atmospheric responses in the lower and upper troposphere, respectively. Using the Community Earth System Model version 1.2 with a slab ocean configuration, we investigate the NAMR’s seasonal impacts by simulating scenarios with and without the mountain range. Our findings reveal that the NAMR induces contrasting responses in sea surface temperature (SST) and precipitation off California in different seasons, indicating different underlying mechanisms. Through analysis of large-scale circulation and local energy budgets, we find that in summer, the NAMR reinforces the North Pacific High causing SST cooling and drying off California. This cooling propagates to the equatorial Pacific via anomalous northeasterlies, influencing the Intertropical Convergence Zone and initiating a climatic signal through the Pacific Meridional Mode, which crosses the equator and affects Southern Hemisphere temperatures. In winter, the NAMR reduces wind speed and evaporation, leading to SST warming off California, amplified by SST-cloud feedback. In the upper troposphere, we observe seasonal shifts in jet stream patterns: during winter, a weakened, equatorward-shifted jet over the Pacific and a strengthened, poleward-shifted branch over the Atlantic; in summer, the jet stream intensifies over and downstream of the mountains while weakening upstream. Our research highlights distinct seasonal mechanisms by which the NAMR influence climate patterns, linking mid-latitude climate variations to equatorial, cross-hemispheric and global changes.